Alternating-current generator.



NO. 789,476. PATENTBD MAY 9, 1905. E. P. w. ALEXANDERSON.

ALTBRNATING CURRENT GENERATOR.

APPLIOATION FILED AUG. 19, 1904.

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PATENTED MAY 9, 190.5.

E. F. W. ALBXANDERSON.

ALTERNATING CURRENT GENERATOR.

APPLIOATION FILED AUG. 19, 1904.

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PATENT OFFICE.

ERNST F. ALEXANDERSON, OF SCHENECTADY, NEW YORK, ASSIGNOR TO GENERALELECTRIC COMPANY, A CORPORATION OF NE YORK.

ALTERNATlNG-CURRENT GENERATOR.

SLEECIFICATION forming part of Letters Patent No. 789,476, dated May 9,1905.

Application filed August 19,1904. Serial No. 221,322.

Ill) all when 71!; may concern:

Be it known that I, ERNs'r F. W. ALEXAN- DERSON, a subject of the Kingof Sweden and Norway, residing at Schenectady, county of Schenectady,Stateof New York, have invented certain new and useful Improvements inAlternating-Current Generators, of which the following is aspecification.

Mv invention relates to self-exciting alter- O nating-cu rrentgenerators of the type in which a portion of the armature-current isrectified and supplied to the field-winding. Numerous machines of thistype have been devised heretofore; but in operation such severe sparking5 was produced at the commutator or rectifier as to render such machinesas heretofore designed unsatisfactory for widely varying loads. Ihavesucceeded, however, in producing a sel f-exciting alternating generatorwhich automatically compensates for varying loads and power factors andwhich runs sparklessl y under all conditions of operation. I believethat the sparking which has hitherto rendered such machines practicallyinoperative for va- 5 rying loads has been due to two causes. In

the first place the circuit from which the exciting-current has beenderived has heretofore always been an inductive circuit in which thecurrent usually had to pass the secondary of some kind of seriestransformer if compounding was desired. The secondary of the seriestransformer in particular acts like a reactance, and the result is thatif the exciting-circuit is broken in commutation violent sparking isproduced, owing to its self-induction. In the second place thefieldwinding itself is of course highly self-inductive, and opening itscircuit in commutation is consequently attended with violent sparking. Ibelieve that one 4 or both of these causes of sparking have been presentin all self-exciting alternating-current generators as heretoforedesigned, while by my invention 1 eliminate both causes of sparking, andthereby obtain entire freedom from sparks with widelywarying loads andpower factors.

One feature of my invention accordingly consists in rendering theexciting-circuit practically non-inductive. 1 accomplish this by placingin series with the potential-trans- 5 former or other source of constantexcitation a non-inductive resistance and connecting the secondary ofthe series transformer, which compensates for the varying loads andpower factors, across a portion or the whole of this non-inductiveresistance. By this arrangement two advantageous results are obtained.In the first place the exciting-circuit is made non-inductive, so thatit can be broken in commutation with practically no sparking, and in thesecond place perfect regulation of current in the exciting-circuitrelative to that in the main circuit is obtained that is to say, whilein an arrangement in which the secondary of potential and seriestransformers are connected in series the series transformer tends at alltimes to overpower the secondary of the potential-transformer, so as todetermine at all times the current flowing in the secondary circuit. Bymeans of my invention the secondary of the series transformer actsmerely to impress upon the terminals of a non-inductive resistanceconnected in the exciting-circuit a potential varying in magnitude andphase with the currentin the main circuit.

A second feature of my invention consists in so arranging the commutatorand its connections that the field-winding is never opencircuited, butis merely transferred from one phase of the exciting-circuit to anotherat a time when the voltages of the two phases are practically equal.Thus the field-circuit is never open, but its self-induction acts merelyto maintain a practically constant currentflow in the field. lurthermore, I so arrange the commutator that a wide insulating-spaceexists between the active segments, so that in case there is a tendencyto spark from any cause, such as improper adjustment of the brushes,flashing over from one segment to another is rendered impossible.

My invention will best be understood by reference to the accompanyingdrawings, in which 9 5 Figure I shows diagrammatically a twophasealternating-current generator provided with compounding transformers anda rectifier arranged in accordance with my invention. Fig. 2 is anexplanatory diagram showing the manner in which the current isrectified. Fig. 3 is an explanatory diagram of the compounding forvarying power factors. Fig. 4 is a simplified diagram of the connectionsof the compounding transformers. Fig. 5 shows a three-phase alternatorprovided with a rectifier and compounding transformers in accordancewith my invention. Fig. 6 is a diagram showing a rectified voltage for athree-phase machine; and Fig. 7 shows an arrangement of the brusheswhich may be sometimes used to avoid sparking.

Referring first to Fig. 1, A represents an armature of analternatingcurrent generator whiclrfor the sake of simplicity I haveshown L as a Gramme ring. It will be understood, however, that inpractice the usual distributed drum-armature winding placed in slots orholes in a laminated armaturebody would be employed. The terminals ofone phase are indicated by the letters a a and of the other phase by theletters a? (f.

F represents the revolving field structure, which carries a novel formof two-part commutator C. ()n the commutator bear two pairs of brushesor sets of brushes 5 b and 7) 7). These brushes are displaced from eachother by ninety electrical degrees that is, by an amount equal to theangular displacement of the phases of the armature-winding. Thecommutator is provided with two conducting-segments c 0, the length ofeach of which is approximately equal to the displacement of the brushesthat is, to ninety electrical degrees. The portions of the commutatorbetween the segments are made of insulating material. A source oftwo-phase current, as will be hereinafter described, is connected to thetwo pairs of brushes.

It will be seen that as one pair of brushes leaves the two segments theother pair reaches the segments, so that the circuit of thefieldwinding, which is connected to the two segments, is never opened,but is simply trans ferred "from one phase to the other. This transfertakes place every ninetydegrees, and if the brushes are properly placedthe resultant potential impressed upon the field-winding F will be asindicated in full lines in Fig. 2. In this figure, 1 represents thepotential of one phase, which is connected to the brushes 6 7/, and 2represents the potential of the other phase, which is connected to thebrushes 5 W. If the brushes are properly placed, the transfer from onephase to the other may occur at the point of intersection of the twocurves, as indicated in the figure. At this point of intersection thetwo phases are at precisely the same potential, and consequently if thetransfer could be made instantaneously at this point absolutely nosparking could be produced. In practice of course it is necessary thatboth pairs of brushes should be in contact with the same segment formore than an instant, since some contact-surface is required between thebrushes and segments to carry the field-current, so that one brush mustpass partly or wholly onto a segment before the other brush entirelyleaves. During this instant one phase is short-circuited on the other;butl reduce the duration of this short-circuit to its minimum value bymaking the segments approximately equal in length to the brushdisplacement, so that one brush begins to leave the segment at theinstant the other brush begins to engage it. \Vith this arrangement theduration of the short-circuit may be made so small that in practice nosparking is visible. Furthermore, owing to the exceedingly greatdistance between segments flashing over on the commutator is impossible.

It will be seen from the drawings that one set of brushes 1/ 7/ isconnected, through the resistance R to the secondary of thepotentialtransformer P. The resistance It is inserted in the circuit forthe purpose that has been heretofore ex1')lained-viv 0., to render theexciting-circuit non-inductive. The presence of this resistance Renables the secondary circuit of the transformer P to be broken by thebrushes Z) 7/ with practically no sparking. In order to compoundproperly for varying loads and power factors, I have shown a seriestransformer S, with its secondary con nected across the terminals ofresistance R Connecting the series transformer in shunt to theresistance in this manner not only leaves the exciting-circuitnon-inductive, butalso insures proper regulation of the current in theexciting-circuit relative to the current inthe main circuit. As has beenmentioned heretofore, proper regulation is not obtained where thesecondariesof series and potential translbrmers are connected in seriesin the closed circuit, as in the ordinary arrangement. The reason forthis has been briefly stated, but will appear more clearly from thefollowing. \Vhere the secondary of a series transformer is included in aclosed circuit, the current-flow in that closed circuit will beabsolutely determined by the current in the circuit of the primary ofthe series transformer and the ratio of transformation. Thus if a seriesand potential transformer have their secondaries simply connected inseries the series transformer will determine the flow of current in thesecondary circuit and the potential-transformer will have no effect.This will be the case unless the potential-transformer is madesulficiently strong to overpower the series transformer-that is, toforce sufficient current through it to saturate its core. If this isdone, the series transformer is of no effect. The above difiiculty maybe somewhat obviated by forming a series transformer with an openmagnetic circuit, so that its leakage-current is high; but thisarrangement produces an inefficient transformer. Moreover, the secondaryof the series transformer at all times acts as a reactance in thesecondary circuit, thereby rendering it highly inductive, so as to makeit impossibie to obtain satisfactory commutation. By connecting theseries transformer in shunt to the non-inductive resistance, however,the above difliculty is wholly removed, while at the same time standardand efficient transformers may be employed. \Vith this arrangement thesecondary circuit of the potential-transformer is closed through anon-inductive resistance and the field, while the secondary of theseries transformer is closed through the resistance. The variation incurrent in the primary winding of a series transformer produces avariation of current in the shunt resistance. This variation of currentthrough the resistance produces a variation of potential at theterminals of the resistance. In other words, the series transformer actssimply to impress upon the field-circuit a potential in series with thatof the potential-transformer and varying with the current in thecircuit.

It will be seen from Fig. 1 that the primary of potential-transformer Pis connected in one phase, while the primary of the series transformeris connected in the second phase. For a non-inductive load, therefore,when the current in each phase is in phase with its potential thesecondary voltages of the two transformers will be ninety degrees out ofphase. This is indicated in Fig. 3, in which the line p indicates themagnitude and phase of the secondry voltage obtained from thepotential-transformer. The line s represents the magnitude and phase ofthe potential obtained from the secondary of the series transformer fora non-inductive load of agiven amount. The vector sum of these twolines, which is represented by c, represents the value of the potentialimpressed upon the brushes 5 7). Now assume that the load remainsconstant in amount, but varies in phase so as to lag forty-live degrees.The secondary voltage of the series transformer will be represented bythe line 8, and the resultant electromotive force impressed upon thebrushes will be represented by the line a. It will be seen that thisline is greater than the line 0. Thus the electromotive force impressedupon the brushes is increased as the power factor decreases. iVith apower factor of zero the secondary of the series transformer will berepresented by the line s and the potential impressed upon the brusheswould be the algebraic sum of the lines p and 8 From this it will beseen that by properly proportioning the potential and seriestransformers the voltage impressed upon the field-winding may beproperly varied to compensate for varying power factors. Similarly, itis evident that if the line 8 or s in Fig; 3 be increased or decreasedthe length of line a or a will be also increased or decreased. Thus thisarrangement automatically compounds for varying load as well as forvarying power factor.

The potential-transformers P and S form a similar arrangement for thesecond pair of brushes 1/ 7/, so that a two-phase excitation isimpressed upon the two of brushes, so as to give an impressedelectromotive force on the field, as shown in Fig. 2. It should beunderstood that Fig. 2 represents the potential impressed upon the fieldand not the current. Since the self-induction of the fieldcircuit isvery great, there is a tendency to smooth out even the smallfluctuations in impressed voltage shown in Fig. 2, so that the currentcurve would be represented by a nearly straight line.

Fig. 5 shows a similar arrangement adapted to a three-phase alternator.Three brushes are employed, which are displaced one hundred and twentyelectrical degrees, and each contact-segment is approximately onehundred and twenty electrical degrees in length, or, in other words, itslength is equal to the angular displacement between the several phasesof the armature. Three series transformers S, S and S are employed inthe three phases of the armature circuit, and three potentialtransformers P P l? are shown connected in delta. The secondaries of thepotential-transformers are also connected in delta. I have shown meansfor varying the phase of the voltage derived from the secondary delta,so that the desired difference of phase between the voltage taken fromany phase of the delta and the voltage impressed upon the resistance inseries therewith by the series transformer may be readily adjusted toits proper value. Furthermore, I have shown the resistance shunting theseries transformer variable in amount. By this means the relative valuesof the voltages from the potential and series transformers may beadjusted. I have also shown the amount of resistance in series with thefield as variable. By this means the field strength may be variedmanually, as is commonly done with alternators as ordinarilyconstructed. The resistances R, R, and R take the place of the usuallieldrheostat employed with a generator.

The action of the commutator is shown in Fig. 6, the point at which thetransfer is made from one phase to the other being the point ofintersection of the curves, as was the case in the two-phasearrangement.

So far the question of the shifting of the point of intersection of twowaves of different phases has not been considered. By referring to Fig.3, however, it will be seen that a change either in the magnitude or inthe phase of the current in the arn1ature-circuit that is, a variationin the length or position of the line .s will produce a change not onlyin the length but also in the position of the line 6 relative to theline 7). The line 1), which represents in phase as well as in magnitudethe voltage of the armature-clrcuit, is lixed with reference to theposition of the field structure in space if the e'li'ect of armatnrereaction be for the moment neglected, which means that it is fixed withrespect to the position in space of the commutator-segments 0 0. Inother words, the variations in the magnitude and phase of thearmaturecurrent shift the potentials impressed upon thecommutator-brushes with respect to the point of commutation.Consequently commutation no longer occurs after such a change at thepoint of intersection of the two current waves, and this change isproductive of a tendency to spark. I have found that the effect ofarmature reaction, which shifts the field flux relative to the fieldstructure. is to counterbalance somewhat this shifting tendency, so thatin a machine properly designed and with the correct relative proportionsof the potential and series transformers the shifting of the point ofcommutation with respect to the impressed voltage is too small toproduce sparking of any consequence. In case the various parts are notproperly proportioned, however, and more or less sparking occurs withchange of the load the arrangement shown in Fig. 7 may be utilized toreduce the sparking to a neglibible amount. In the arrangementshown inFig. Teach brush 7/ is composed of a plurality of brushes displaced by asmall amount around the commutator. The several brushes are connected bya resistance r. This form of brush and its principle of operation arewell known and need no explanation.

For the sake of simplicity I have shown my invention as applied to abipolar machine. It will be understood, however, that it is applicableto a machine with any number of poles. The commutator-segments in eachcase will be equal in number to the number of poles, and the length ofthe commutatorsegments will always be approximately equal to the angulardisplacement between the several phases, it being understood that-thislength is measured in electrical and not in physical degrees.

Although I have shownthe constant excitation derived indirectly from thearmaturewinding through potentialtransformers, it will be understoodthat any other suitable source of constant potential, such as winding ofthe machine itself, may be utilized in place of thepotential-transformers.

I do not desire to limit myself to the particular construction andarrangement of parts shown; but I aim in the appended claims to coverall modifications which are within the scope of my invention.

\Vhat I claim as new, and desire to secure by Letters Patent of theUnited States, is

1. In combination with a polyphase alternating-current generator havinga fieldwind ing provided with a commutator, a source of polyphasepotential connected to said commutator, non-iiuluctive resistancesconnected in series with said source, and series transformers havingtheir primaries connected in series with the several phases of thearmature and having their secondaries connected in shunt to saidresistances.

2. In combination with a polyphase alternating-current generator havinga field-winding provided with a con'imutator, a source of polyphasepotential connected to said conu'nutator, non-inductive resistancesconnected in series with said source, and series transformers havingtheir primaries connected in series with the several phases of thearmature and having their secondaries connected in shunt to saidresistances, said transformers being connected to produce when the loadon the generator is non-inductive a potential at the terminals of theshunting resistance substantially ninety degrees out of phase with thepotential of that phase of said polyphase source in series with whichsaid resistance is connected.

3. In combination with a polyphase alternating-current generator havinga field-winding provided with a commutator, potentialtransformers andseries transformers energized from the armature-clrcuit and having theirsecondaries connected to said COITlIl'lUttttor, and non-inductiveresistances connected in series with the mtential-transformers and inshunt to the series transformers.

4:. In combination with a polyphase alternating-current generator havinga field-winding provided with a commutator, a source of polyphasepotential connected to said commutator, resistances connected in serieswith the several phases of said source, and series transformers eachhaving its secondary connected in shunt to one of said, resistances andhaving its primary connected in the armature-circuit to produce, whenthe load on the generatm' is non-iiuluctive, a secondary voltage at anangle of amn'oximately ninety degrees to the voltage of that phase ofthe polyphase source in series with which said resistance is con nectcd.

In combination with a polyphase alternating-current generator having afield-winding provided with a commutator, a source of polyphasepotential connected to said commutator, resistances connected in serieswith the several phases of said source, series transformers each havingits secondary connected in shunt to one of said resistances and hayingits primary connected in the armature-circuit to produce, when the loadon the generator is non-inductive, a secondary voltage at an angle ofapproxin'iately ninety degrees to the voltage of that phase of thepolyphase source in series with which said resistance is connected, andmeans for varying the magnitude of said angle.

6. In combination with a polyphase alternating-current generator havinga field-winding provided with a commutator, a source of polyphasepotential connected to said commutator, non-inductive resistancesconnected in series with said source, series transformers having theirprimaries connected in series with the several phases of the armatureand having their secondaries connected in shunt to said resistances, andmeans for varying the amount of said resistances.

7. In combination with a polyphase alternatingcurrent generator having afield-winding provided with a commutator, means for impressing on saidcommutator a polyphase constant voltage derived from the armature,resistances in series with the several phases of said voltage, andseries transformers each having its primary connected in series with onephase of the armature and its secondary in shunt to one of saidresistances.

8. In combination with a polyphase alternating-current generator havinga field-winding provided with a commutator, means for impressing on saideol'nmutator a polyphase constant voltage derived from the armature,resistances in series with the several phases of said voltage, seriestransformers each having its primary connected in series with one phaseof the armature and its secondary in shunt to one of said resistances,and means for varying the amount of said resistances.

9. In an alternating-current generator, a revolving field-magnet, acommutator carried thereby having two segments per pair of poles,brushes bearing on said commutator, and means for impressing polyphasevoltages on said brushes, the length of each commutatorsegment beingapproximately equal to the angular displacement between the phases ofsaid voltages.

10. In an alternating-current generator, a poly phase armat u rewinding,a revolving fieldmagnet, a commutator carried by said magnet having twosegments per pair of poles, the length of each segment beingapproximately eq ual to the angular displacement of the phases of thearmature, symmetrically disposed brushes bearing on said commutator andcorresponding in number to the phases of the armature, and means forimpressing on said brushes a polyphase excitation derived from thearmature.

11. In an alternating-current generator, a revolving field-magnet, acommutator carried thereby having two segments per pair of poles, andmeans for impressing a polyphase excitation on said commutator, thelength of the segments of said commutator being approxi mately equal tothe angular displacement be tween the phases of said excitation.

12. In an alternating-current generator, a revolving field-magnet, asource of polyphase excitation therefor, a commutator carried by saidfield-magnet having two segments per pair of poles, the length of eachsegment be ing approximately equal to the angular displacement betweenthe phases of said excitation, and symn'ietrically displaced brushesbearing on said commutator and connected to said source of excitation.

13. In an alternating-current generator, a pol yphase armature-w inding,a revolving fieldmagnet, a commutator carried by said magnet having twosegments per pair of poles, the length of each segment beingapproximately equal to the angular displacement of the phases of thearmature, symmetrically disposed brushes bearing on said commutator andcor responding in number to the phases of the armature, means forimpressing on said brushes a polyphase excitation derived from thearmature, and means for varying the voltage impressed on said brusheswhen the magnitude or phase of the armatLire-current varies.

14. In an alternating-current generator, a revolving field-magnet, acommutator carried thereby having two segments per pair of poles,brushes bearing on said commutator, means for impressing polyphasevoltages on said brushes, the length of each commutatorsegment beingapproximately equal to the angular displacement between the phases ofsaid voltages, and means for varying said voltages when the load on thegenerator varies in magnitude or in phase.

In witness whereof I have hereunto set my hand this 18th day of August,1904.

ERNST ll. W. ALEXANDERSON. \Vitnesses:

BENJAMIN B. HULL, HnLnN ORFORD.

